The present device is generally related to materials testing equipment, more specifically to such equipment used for measuring the resistance of selected materials or structures to impact damage, and more particularly to the resistance of roofing structures and materials to environmental impact damage, for example from hail, falling branches or the like.
Underwriter's Laboratories (UL) has developed a test, known as UL 2218 for studying the resistance of various roofing materials to impact damage. A main objective of the test is to evaluate the resistance of target roofing structures to hail damage. A steel ball is dropped down a tube made of PVC plastic or the like, the pipe having a diameter large enough to accommodate the free fall of the steel ball. Ideally, the steel ball falls down the centerline of the pipe. The length of the pipe varies, depending on the diameter of the ball, with lengths of from 12-20 feet being customary. Impact damage by the ball falling on test roofing structure has been found to correlate with actual hail damage.
A steel fork having a pair of parallel tines spaced narrower than the diameter of the ball is transversely and slidably located near an upper end of the pipe, and is laterally retracted to initiate the ball drop. However, in practice, variations in the retraction of the fork, and the potential for the falling ball to contact the inner surface of the pipe create the possibility of inconsistent test results. Accordingly, there is a need for an improved ball drop test apparatus.